The Universe

Evolution of the Universe

Who first discovered the Doppler effect for light from an astronomical source?

The first astronomer to observe a Doppler shift from a distant object was Vesto Melvin Slipher in 1912. Slipher (1875–1969) used telescopes to photograph and study large, fuzzy patches of gas and dust, called nebulae, which were thought to be within the Milky Way galaxy. Much to everyone’s surprise, Slipher found that many of these patches were made of stars, which suggested that they could be distant galaxies like the Milky Way.

In 1903 Slipher accepted a scientific position at the Lowell Observatory in Flagstaff, Arizona. He was brought to Flagstaff by the astronomer Percival Lowell (1855–1916) to investigate these nebulae. Lowell thought that some of these cloudlike structures, particularly the ones that had spiral patterns, might be the beginnings of other solar systems within our galaxy. Slipher’s job was to study the spectra of the nebulae so they could be carefully analyzed.

Studying the spectrum of the Andromeda nebula, Slipher discovered it did not match the spectrum of any known gas. Rather, it was more like the spectrum made by starlight. Even more amazing, the colors of that starlight appeared to be blueshifted. Slipher concluded that the Andromeda nebula was actually moving toward Earth at a remarkable speed of about half a million miles per hour. Over the following years, Slipher analyzed the spectra of twelve other spiral nebulae. He found that some were moving toward Earth and some were moving away. Furthermore, these nebulae were moving at remarkable speeds of up to 2.5 million miles per hour (1,100 kilometers per second). He concluded that these objects were not nebulae at all, but entire systems of millions or billions of stars, so distant that they had to be galaxies. Slipher’s pioneering work was later confirmed by Edwin Hubble, who used Cepheid variables as standard candles to prove that the great nebula in Andromeda was in fact the Andromeda Galaxy.